Radium
Radium is a chemical element with atomic number 88 and chemical symbol Ra, belonging to the alkaline earth metals group in the periodic table.[1][2] It appears as a silvery-white metal that rapidly tarnishes in air due to its high reactivity, and it is extraordinarily radioactive, with alpha, beta, and gamma emissions from its isotopes causing intense ionization.[2] The most stable isotope, radium-226, has a half-life of approximately 1,600 years and decays into radon gas, contributing to its natural occurrence in trace amounts within uranium ores such as pitchblende, from which about 1 gram is extractable per 7 tons of ore.[1][2] Discovered in 1898 by Marie and Pierre Curie through laborious chemical separations from pitchblende residues, radium's isolation marked a pivotal advancement in understanding radioactivity, earning the Curies the Nobel Prize in Physics in 1903.[3][2] Initially celebrated for its luminous properties and potential in medical radiotherapy, radium's unchecked use in consumer products like luminescent paints and tonics led to severe health consequences, including anemia, bone necrosis, and cancers, as evidenced by cases among factory workers exposed to its emanations.[4][5] Its causal role in radiation-induced pathologies underscored the double-edged nature of radioactivity, shifting applications toward controlled neutron sources and away from direct human exposure due to the empirical link between radium ingestion or inhalation and osteosarcoma development.[4][6]
Physical Properties
Bulk Properties
Radium is a dense, silvery-white alkaline earth metal that rapidly tarnishes in air and self-heats due to radioactive decay.[7] At standard temperature and pressure (20 °C, 101.325 kPa), it exists as a solid.[7] Its density is 5 g/cm³, comparable to that of barium but lower than calcium due to increasing atomic volume down the group.[7] [8] The melting point of radium is 696 °C (969 K), and the boiling point is approximately 1500 °C (1773 K); these values are based on early experimental measurements extrapolated with group trends, as pure samples are scarce.[7] Radium crystallizes in a body-centered cubic lattice (space group Im-3m) with a lattice constant of 514.8 pm, consistent with the structures of barium and other heavy alkaline earth metals.[9]| Property | Value | Unit |
|---|---|---|
| Density | 5 | g/cm³ [7] |
| Melting point | 696 | °C [7] |
| Boiling point | 1500 | °C [7] |
| Crystal structure | Body-centered cubic | [9] |
| Lattice constant (a) | 514.8 | pm [9] |
Isotopes
Radium (atomic number 88) has 34 known isotopes, ranging from ^{201}Ra to ^{234}Ra, all of which are radioactive and unstable, with half-lives spanning from fractions of a second to approximately 1600 years.[11] The element possesses no stable isotopes, and the majority decay via alpha emission, though some shorter-lived variants undergo beta decay or electron capture.[12] Among these, ^{226}Ra is the longest-lived and most abundant naturally occurring isotope, with a half-life of 1600 years, decaying primarily by alpha emission to ^{222}Rn.[13] Four isotopes occur naturally as intermediates in the actinium (^{235}U), thorium (^{232}Th), and uranium (^{238}U) decay series: ^{223}Ra (half-life 11.43 days, alpha decay), ^{224}Ra (half-life 3.6319 days, alpha decay), ^{226}Ra (half-life 1599 years, alpha decay), and ^{228}Ra (half-life 5.75 years, beta decay).[14] These isotopes are present in trace amounts in uranium- and thorium-bearing minerals, with concentrations typically on the order of 1 part per trillion in the Earth's crust for ^{226}Ra.[15] Artificially produced isotopes, such as ^{225}Ra (half-life 14.9 days), have been synthesized in particle accelerators or nuclear reactors for research, including potential medical applications like targeted alpha therapy due to their emission of high-energy alpha particles.[12] The table below summarizes key properties of the principal naturally occurring radium isotopes:| Isotope | Half-life | Primary decay mode | Parent nuclide in chain | Daughter nuclide |
|---|---|---|---|---|
| ^{223}Ra | 11.43 days | Alpha | ^{227}Ac (U-235 series) | ^{219}Rn |
| ^{224}Ra | 3.6319 days | Alpha | ^{228}Th (Th-232 series) | ^{220}Rn |
| ^{226}Ra | 1599 years | Alpha | ^{230}Th (U-238 series) | ^{222}Rn |
| ^{228}Ra | 5.75 years | Beta minus | ^{232}Th (Th-232 series) | ^{228}Ac |
Chemical Properties
Reactivity
Radium, the heaviest stable member of the alkaline earth metals, displays high chemical reactivity characteristic of group 2 elements, though relativistic effects from its high atomic number result in a first ionization energy (509.3 kJ/mol) higher than barium's (502.9 kJ/mol), potentially moderating its reactivity relative to group trends.[18][1] Despite this, radium tarnishes rapidly upon exposure to air, preferentially reacting with nitrogen over oxygen to form a black surface layer of radium nitride via the reaction $3\mathrm{Ra} + \mathrm{N_2} \rightarrow \mathrm{Ra_3N_2}.[19] This nitride formation, observed in pure samples, contrasts with the oxide layers typical of lighter congeners and arises from the greater thermodynamic stability of radium nitride under ambient conditions.[19] The element's intense radioactivity contributes to self-heating (approximately 0.1 W/g for ^{226}Ra), which may accelerate surface reactions and impart luminosity, but the primary chemical driver is its large atomic radius (221 pm) facilitating easy loss of the 7s^2 electrons.[20][21] In contact with water, radium decomposes it vigorously to yield radium hydroxide and hydrogen gas: \mathrm{Ra + 2H_2O \rightarrow Ra(OH)_2 + H_2}.[18] This reaction proceeds more rapidly than for barium due to radium's lower lattice energy and increased metallic character, though direct comparisons are complicated by radium's scarcity and radiation-induced alterations; some accounts describe the process as less explosive than barium's but still exothermic and gas-evolving.[18][19] Radium also reacts with dilute acids, such as hydrochloric acid, to produce soluble radium salts and hydrogen: \mathrm{Ra + 2HCl \rightarrow RaCl_2 + H_2}, mirroring barium but with potentially enhanced solubility of products owing to the larger Ra^{2+} ion (162 pm ionic radius).[20][22] Radium exhibits reactivity toward halogens, forming dihalides like radium chloride (RaCl_2) upon heating in chlorine gas, though these compounds hydrolyze readily in moist air.[20] It does not displace hydrogen from stronger bases like sodium hydroxide solutions under standard conditions, consistent with its position in group 2.[18] Overall, radium's reactivity supports its +2 oxidation state exclusively in known compounds, with no stable +1 or higher states observed, underscoring its behavior as a typical s-block metal despite anomalies from relativistic stabilization of the 7s electrons.[1] Experimental data remain limited, as radium's short half-lives (e.g., 1600 years for ^{226}Ra) and alpha emission necessitate handling in microgram quantities, often leading to inferences from barium analogs adjusted for periodic trends.[21]Compounds
Radium forms predominantly ionic compounds in the +2 oxidation state, analogous to other alkaline earth metals, though its intense radioactivity leads to rapid radiolytic decomposition, often causing discoloration from white to yellow or dark hues over time.[23] These compounds exhibit properties influenced by radium's position in the periodic table, with solubility trends decreasing down group 2; however, radium salts are generally more soluble than barium counterparts due to weaker lattice energies from the larger ionic radius of Ra²⁺ (162 pm vs. 135 pm for Ba²⁺).[24] Radium halides, such as radium chloride (RaCl₂) and radium bromide (RaBr₂), are notable for their relatively high water solubility, facilitating historical isolation and applications. RaCl₂ crystallizes as a dihydrate from aqueous solutions and exhibits blue-green luminescence upon heating, with solubility decreasing compared to lighter group 2 chlorides but sufficient for radon gas production via emanation for early radiotherapy.[25] RaBr₂ demonstrates even greater solubility (approximately 70 g/100 g water at 20°C), a melting point of 728°C, and sublimation around 900°C, rendering it preferable for fractional crystallization in radium purification from uranium ores.[26][23] In contrast, radium sulfate (RaSO₄) possesses extremely low solubility, among the least soluble sulfates known, with a solubility product enabling its precipitation for radium separation from complex mineral matrices.[27] Similarly, radium carbonate (RaCO₃) and radium phosphate exhibit low solubilities, exploited in purification schemes where sulfate or carbonate precipitation isolates radium from barium and other interferents.[23] Radium nitrate (Ra(NO₃)₂) is highly soluble, forming colorless solutions used in early radium preparations.[24]| Compound | Formula | Key Property | Solubility in Water |
|---|---|---|---|
| Radium chloride | RaCl₂ | Forms dihydrate; luminescent | Soluble[25] |
| Radium bromide | RaBr₂ | Higher solubility than chloride; mp 728°C | ~70 g/100 g at 20°C[26] |
| Radium sulfate | RaSO₄ | Least soluble sulfate | Very low[27] |
| Radium carbonate | RaCO₃ | Used in precipitation | Low[23] |